Prepare the Tuna Mixture: In a medium bowl, combine the drained tuna, mayonnaise, Dijon mustard (if using), celery, onion, garlic powder, salt, and pepper. Mix until well combined.
Toast the Bread (Optional): Lightly toast the bread slices for added texture and flavor.
Assemble the Sandwich:
Place a lettuce leaf on one slice of bread.
Spread half of the tuna mixture evenly over the lettuce.
Top with tomato slices, if desired.
Cover with another slice of bread.
Repeat: Repeat the process for the second sandwich.
Serve: Cut the sandwiches in half and serve immediately.